To assess the production of the nonessential amino acid tyrosine in preterm infants, we estimated the activity of phenylalanine hydroxylase (PAH) in three groups of infants by measuring the conversion of phenylalanine to tyrosine, using a model based on a primed constant 200-min intravenous infusion of (2H5]Phenylalanine. We determined the isotopic enrichments of (2H5]phenylalanine and (2H4]tyrosine by selected-ion-monitoring gas chromatography-mass spectrometry (GCMS). Group 1 (n = 7, mean gestational age 29.7+ 15 wk, mean birth weight 1.4 + 0.4 kg) was studied during the first 4 d of life before initiation of amino acid nutrition. Group 2 (n - 7, mean gestational age 29.7 + 1.5 wk, mean birth weight 1.4 + 0.4 kg) was studied at 16 d of life after receiving amino acid nutrition. Group 3 (n = 4, mean gestational age 28.5 + 0.9 wk, mean birth weight 1.1 + 0.1 kg) was studied during the first 4 d of life after receiving amino acid nutrition. Calculated from the observed enrichments, phenylalanine conversion to tyrosine was 5.9 + 2.6, 19.4 + 8.8, and 11 + 1.8 umol kg-1 h-1 in groups 1, 2, and 3, respectively. The rate of conversion of phenylalanine to tyrosine increased significantly after initiation of amino acid nutrition. We conclude that preterm infants are capable of converting phenylalanine to tyrosine. Pro vision of phenylalanine in the context of parenteral amino acid nutrition solution accelerated PAH conversion of phenylalanine to tyrosine, suggesting that the enzyme system is capable of responding normally to provision of substrate. Further, under the circumstances of parenteral phenylalanine administration, PAH conversion of phenylalanine to tyrosine occurred at a rate equivalent to estimated tyrosine needs in preterm infants. Therefore, we conclude that tyrosine is not a conditionally essential amino acid for preterm infants under these conditions.